Abdelbaset al-Megrahi was convicted of killing 270 people when Pan Am flight 103 exploded over Lockerbie.

The Scottish government has rejected the claims by the group.

Justice for Megrahi wants the International Association of Prosecutors (IAP) to look at how Mr MacAskill and Mr Mulholland dealt with allegations against the Crown Office and police which handled the Lockerbie case.

The IAP, which was formed as a non-political organisation in 1995, aims to raise standards of professional conduct and ethics for prosecutors across the world, while promoting the rule of law, fairness, impartiality and respect for human rights.

The association can consider whether the complaint amounts to a breach of its standards.

The move by Justice for Megrahi is part of a wider campaign for an independent inquiry into the 1988 conviction of Megrahi, who died last year having been released from a Scottish jail in 2009.

MSPs on Holyrood's justice committee are continuing to consider the group's ongoing petition.

'Absolutely false'

It alleges four breaches, covering independence, impartiality, professional conduct and role in criminal proceeding.

A Scottish government spokeswoman pointed out that Megrahi was convicted in a court of law and a court was the only appropriate forum for considering all the evidence in the case and "determining his guilt or innocence".

She added: "In relation to the reporting of Mr MacAskill to the IAP, the cabinet secretary [Mr MacAskill] has not had any involvement in decisions made by the Crown Office and the police in relation to their consideration of the al-Megrahi case nor their on-going investigation into the Lockerbie bombing. Indeed, the cabinet secretary for justice is not a prosecutor."

A Crown Office spokesman said allegations made by Justice for Megrahi were being considered by Chief Constable Patrick Shearer, the former top police officer in the old Dumfries and Galloway Constabulary.

He added that it was "absolutely false" that the Crown Office and Procurator Fiscal Service had instructed him not to investigate any of their allegations.

But in its submission to Holyrood's justice committee, group secretary of Justice for Megrahi, Robert Forrester, stated: "At a time when there is growing concern about the Crown Office and police handling of the whole Lockerbie inquiry, this latest international complaint makes it even more important that our petition remains a live issue within the Scottish Parliament."

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